This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
One of the most basic requirements for any cellular system is the possibility for a User Equipment (UE) to initially request a connection setup to a network side (e.g., a Base Station (BS) or an eNodeB (eNB) in Long-Term Evolution (LTE) or any other appropriate BS that can guide the UE to establish a connection to the network), commonly referred to as random access. In LTE, the random access procedure comes in two forms, allowing access to be either contention-based or contention-free.
In a contention-based random access procedure, a preamble is randomly chosen by a UE from a preamble group, with the result that it is possible for more than one UE to simultaneously transmit the same preamble (i.e., a contention occurs), leading to a need for a subsequent contention resolution process. The smaller the total number of preambles available in the contention-based random access procedure is, the higher the contention possibility becomes.
For a contention-free random access procedure, the network side has the option of preventing contention occurring by allocating a dedicated preamble to a UE, resulting in contention-free access. This procedure is constrained to a limited amount of available preambles. That is, the smaller the total number of preambles available in the contention-free random access procedure is, the bigger the total number of preambles available in the contention-based random access procedure is, the lower the contention possibility becomes.
The evolving 5th Generation (5G) standard New Radio (NR) is aiming to operate in a wide range of frequencies from below 1 GHz up to 100 GHz. At high frequencies, it is well known that coverage will be an issue. One way to mitigate this is the introduction of a Supplementary Uplink (SUL) carrier which is mainly motivated by improvement of the uplink coverage for NR which is deployed at relative high frequency bands. The SUL carrier is deployed at the low frequency region, for example, at the LTE band. In this way, the uplink coverage of NR bands can be comparable with that in LTE. SUL carrier and NR UL/DL(Down Link) carrier are in the same cell. In this case, the SUL carrier belongs to more like a separate UL configuration. The UE can maintain two UL configurations, but the UE may keep only one UL configuration active. FIG. 1 illustrates the coverage differences between the NR UL carrier and the SUL carrier. The NR UL carrier covers a smaller area than the SUL carrier.
Problems occurs when UEs of the two uplink physical resource configurations (NR UL and SUL) choose the same preamble for contention-based random access request, as there are no contention resolution for such a scenario. Once a grant is issued for only one of the colliding UEs, each of the UEs will take it as a grant for itself and then an error occurs.